This application claims the priority of German Application No. 198 02 484.3, filed Jan. 23, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for manufacturing built-up camshafts and to a system for implementing this process.
In the case of conventionally built-up camshafts, the outside tube diameter, which depends on the dimensions of the bearing shells, must not fall below a defined measurement. This is so that the stiffness of the camshaft as a whole and the resistance to wear of the bearing points of the camshaft are ensured. Simultaneously, the outside tube diameter corresponds to the bore diameter of the cam which is pushed onto the hollow shaft. Since the cam has a predetermined defined course in the transverse direction with respect to the hollow shaft, in order to carry out the operation of the charge cycle valves corresponding to their function, the cam belt (thus the cam section which forms the so-called base circle of the cam) becomes thinner as the outside tube diameter of the hollow shaft becomes larger. If the cam belt now falls below a certain thickness, it is difficult to achieve a lasting joining of the cam on the hollow shaft when a press fit is reached between the hollow shaft and the cam. This is because the cam no longer has a sufficient stiffness for absorbing the joining tension.
For manufacturing the camshaft, a medium outside tube diameter must therefore be maintained, at which, on the one hand, the camshaft and the bearing point have a sufficient stiffness and the bearing point is provided with a sufficient resistance to wear and, on the other hand, the cam belt is still sufficiently strong in order to ensure the capacity of the cam to absorb the joining tension. However, for reasons of space, because of the specific construction of the engine, the engine abutment for the bearing points of the camshaftxe2x80x94the bearing shellsxe2x80x94, is arranged in some engines in a position in which the camshaft is spaced at a distance from the engine at its bearing point. As indicated, for example, in European Patent Document EP 0 328 010 A1, in which the joining of the cams, by the way, takes place with the admission of internal high pressure by means of a lance introduced into the hollow shaft, the bridging of the distance is, as a rule, achieved by the fastening of bearing sleeves on the camshaft at the position of the bearing points. However, the bearing sleeves have the disadvantage that they represent a separate component and therefore require a separate manufacturing. In addition, they must be fine-machined in a manner which requires higher expenditures and considerably increases costs for obtaining a high-quality surface. Further, they must be non-rotatably mounted on the hollow shaft. On the other hand, the bearing sleeves must sometimes have very thin dimensions (approximately 1 mm), whereby a stability on the hollow shaft when loaded in the engine operation virtually becomes non-existent.
From German Patent Document DE 37 04 092 C1, a built-up camshaft is known. Here, a bearing point is shaped out of the hollow shaft by means of an expanding internal high pressure forming. In this process, the hollow shaft, together with the cams to be joined on it, is placed and positioned in a hollow receiving mold of an internal high pressure forming tool consisting of at least two female molds. Then, while the tool is closed and a fluidic high pressure is applied in the hollow shaft, this hollow shaft is expanded. Thus, the hollow shaft is pressed together with the cams at the point where they are located. Simultaneously, the bearing points are widened corresponding to the distance to the abutment to be bridged. In this case, the high-expenditure system is disadvantageous because a press must apply the complete locking pressure for the internal high pressure forming tool or for the projected surface of the workpiece. Furthermore, the hollow shaft material, during the internal-high-pressure-caused widening in the area of the inserted cams, flows in the direction of the joints between the tool and the cams. This causes corresponding accumulations of material in the hollow shaft on both sides of the faces of the cam in the transition area of the cam face to the hollow shaft which push radially to the outside. These accumulations cause tensile stress peaks in the cam which result in an increased wear of the cam track. In addition, as a result of the load cycle in the engine operation, a dynamic moment of force affects the accumulations in the radial and axial direction. This results in a loosening of the cam on the shaft. Overall, because of the above-mentioned problems, the known process and system for manufacturing a built-up camshaft is suitable for vehicle use only to a limited extent, if at all.
It is an object of the invention to provide a process and a system for manufacturing a built-up camshaft by which the operational safety of the camshaft is ensured in a simple manner in any engine operation, and the camshaft can arbitrarily be adapted to abutments which have different positions because of the individual engine construction.
According to the invention, this object is achieved by a process and system for manufacturing built-up camshafts, at least one cam being pushed onto a hollow shaft, after which the hollow shaft is expanded between the two faces of the cam which extend transversely with respect to the longitudinal course of the hollow shaft. At the respective bearing point, by means of a highly pressurized pressure fluid which is delivered by a lance-shaped probe introduced into the hollow shaft, such that, on the one hand, a press fit occurs between the cam and the hollow shaft and, on the other hand, a bulging of the bearing point of the hollow shaft occurs which bridges the distance between the hollow shaft and the abutment, the hollow shaft is sealed off between the expanded points by a sealing arrangement on the probe with respect to the expanding internal high pressure.
As a result of the invention, the outside hollow shaft diameter of the unformed output shaft can be selected largely independently of the bearing diameter. If the outside diameter of the hollow shaft is selected appropriately, it is dimensioned to be so large that the hollow shaft resists bending. This bending resistance is sufficient for any engine operating situation. It is also dimensioned to be so small that, because of a sufficient cam belt thickness, the cam still has enough stiffness that it can absorb the joining tensions during the internal high pressure admission to the hollow shaft, on the one hand, and, on the other hand, without suffering any damage in the engine operation at any load.
Because of the independence of the bearing diameter of the dimensions of the residual hollow output shaft which is adjustable as desired, because of a targeted expanding forming of the bearing point of the hollow shaft by means of internal high pressure generated by way of a lance-type probe placed in the hollow shaft, modern engines of almost any construction can be equipped with a single type of camshaft. In this case, only the bearing point must be formed differently. This can easily be achieved by the variability of the manufacturing by internal high pressure forming. Thus, the camshaft can be almost arbitrarily adapted in a simple fashion to the constructional constraints with respect to the abutment and can be designed in its dimensioning in an operationally safe manner without the requirement of accepting more or less useful compromise solutions as the result of the dependence on the construction of the bearing point. The now possible use of essentially the same camshafts for different engines largely simplifies the manufacturing of the camshafts, especially considering the previous multiplicity of versions of shaft dimensions that were used. This thus considerably reduces the equipment expenditures and costs. With respect to the use of bearing sleeves, the separate component of the bearing sleeve can be completely eliminated. This is because this bearing sleeve can virtually be shaped out of the hollow shaft. This saves additional costs and the expenditures for non-rotatably fastening this bearing sleeve on the hollow shaft. A fastening of this type is particularly difficult in that the manufacturing of separate components as a rule has tolerances which must be taken into account later during the joining. In this case, after concluding the grinding process of the cams and the bearing points, the wall thickness of the bearing sleeve, which is normally low, may be reduced to such an extent that a joining or fastening of the bearing sleeve on the hollow shaft in an operational manner cannot be made possible. Furthermore, because of the construction of the bearing point according to the invention, there is no danger of a detachment in the engine operation. As the result of the use of the lance-shaped probe for the joining of the cam as well as for the construction of the bearing point, by which shapings and expansions of the hollow shaft can be achieved in a targeted manner without any overall stress to the hollow shaft by means of internal high pressure, a die with the pertaining immense equipment expenditures can be eliminated. Only a clamping-in is required for the hollow shaft and the supporting tools for the cams and the bearing points. Because of the targeted use of the internal high pressure, in contrast to the use of a die, ejections of the hollow shaft, which form directly following the cams and which result in a loosening of the respective cam in the engine operation, can be avoided. This contributes considerably to the operational reliability of the camshaft in the engine operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.